In recent years, a ferroelectric thin film used as a capacitor and a piezoelectric element has been widely developed so as to satisfy the requirements for downsizing an electronic device.
Lead zirconate titanate (PZT) is a ferroelectric material that has a perovskite structure and shows an excellent dielectric characteristic. A CDS (Chemical Solution Deposition) method using sol-gel solution has been attracting attention in producing a thin film capacitor where the PZT is used as a material for ferroelectric thin film, as its film formation process is inexpensive and a uniform film composition can be obtained in a substrate plane.
In the formation of such a ferroelectric thin film by the CDS method using a sol-gel solution, it is possible to produce a ferroelectric thin film having crystals with preferential orientation in a (111) plane depending on the (111) axis direction of the lower electrode, by forming platinum (Pt) or iridium (Ir), where a crystal plane is oriented in the (111) axis direction, as the lower electrode on a substrate and forming the ferroelectric thin film on the lower electrode. Since such a ferroelectric thin film having crystals with preferential orientation in the (111) plane has high withstand voltage and life-time reliability, the ferroelectric thin film is preferably utilized in the application such as an IPD (Integrated Passive Device) or a non-volatile memory.
In addition, conventional methods known to have crystals with preferential orientation in the (100) plane or (110) plane on the lower electrode oriented in the (111) axis direction include using a material different from the ferroelectric thin film as a seed layer and introducing a material different from the ferroelectric thin film as a buffer layer so as to suppress the influence of the lower electrode. Since the ferroelectric thin film having crystals with preferential orientation in the (100) plane has a large piezoelectric constant e31, it is preferably utilized in the application such as an actuator. Furthermore, since the ferroelectric thin film having crystals with preferential orientation in the (110) plane has a large dielectric constant, it is preferably utilized in the application such as a capacitor.
There is a disclosure of a method for producing a ferroelectric film as a technique introducing a buffer layer (for example, see Patent Document 1). The method for producing a ferroelectric film includes: a step of forming a base film oriented to a predetermined crystal plane on a substrate; a step of forming a carbon film on the base film; a step of forming an amorphous thin film containing a ferroelectric material on the carbon film; and a step of forming a ferroelectric film on the base film by heating the amorphous thin film to crystallize the amorphous thin film. The ferroelectric film produced by this method is oriented to a crystal plane different from a predetermined crystal plane, and the ferroelectric material is formed from at least one of: the perovskite structure and bismuth layered-structure oxide; superconducting oxide; tungsten-bronze structure oxide; at least one material selected from the group consisting of CaO, BaO, PbO, ZnO, MgO, B2O3, Al2O3, Y2O3, La2O3, Cr2O3, Bi2O3, Ga2O3, ZrO2, TiO2, HfO2, NbO2, MoO3, WO3 and V2O5; a material containing SiO2 in at least one material selected from the group consisting thereof; and a material containing SiO2 and GeO2 in at least one material selected from the group consisting thereof. In Patent Document 1, the crystal orientation of the ferroelectric film formed on the carbon film is controlled by adjusting the thickness of the carbon film formed as a buffer layer. Patent Document 1 shows the control of the crystal orientation that the orientation of the PZT is controlled to have (111) plane+(001) plane orientation when the thickness x of a DLC (diamond like carbon) film which is the carbon film is in the range of 0 nm<x<10 nm, the orientation of the PZT is controlled to have (001) plane orientation when the thickness x of the DLC film is 10 nm, the orientation of the PZT is controlled to have (001) plane+(110) plane orientation when the thickness x of the DLC film is in the range of 10 nm<x<100 nm, the orientation of the PZT is controlled to have (110) plane orientation when the thickness x of the DLC film is 100 nm, and the orientation of the PZT is controlled to have weak (110) plane orientation when the thickness x of the DLC film is larger than 100 nm.
Patent Document 1 also discloses that it is possible to produce a PZT film having (001) orientation on the lower electrode having (111) orientation using a buffer layer where LaNiO3 strongly self-oriented in the (001) direction is stacked on the lower electrode.
However, the method in Patent Document 1 described above requires complicated processes including introduction of the seed layer or the buffer layer. In addition, there is a concern that the presence of such a seed layer or a buffer layer may cause deterioration of characteristics of the ferroelectric thin film, contamination, or the like.
In addition, as a method for controlling the crystal orientation of the ferroelectric thin film, there is a disclosure of a method for controlling the crystal orientation of the ferroelectric thin film including: coating a precursor solution of PZT or PLZT on a platinum substrate where a crystal plane is oriented in the (111) axis direction and heating the substrate to form a ferroelectric thin film, in which the substrate that has been coated with the precursor solution is firstly subjected to a heat treatment in the temperature range of 150° C. to 55° C. to achieve a desired crystal orientation, and is subsequently subjected to the firing in the temperature range of 550° C. to 800° C. and the crystallizing of the precursor to preferentially orient the crystal plane of the thin film in a certain axis direction depending on the heat treatment temperature (for example, see Patent Document 2). In Patent Document 2, the crystal orientation of the ferroelectric thin film is controlled depending on the temperature range of heat treatment corresponding to preliminary firing, thereby forming a ferroelectric film having the controlled crystal orientation on the lower electrode directly without introducing the seed layer or the buffer layer. Patent Document 2 concretely shows the control of the crystal orientation such that the preferential orientation in the (111) plane is achieved by the heat treatment at 150° C. to 250° C., the preferential orientation in the (111) plane and the (100) plane is achieved by the heat treatment at 250° C. to 350° C., and the preferential orientation in the (100) plane and the (200) plane is achieved by the heat treatment at 450° C. to 550° C.